This application claims priority of Japanese Application No. 2001-170869 filed June 6, 2001.
1. Field of Invention
The present invention relates to a control system and to a method of control for a hybrid vehicle which uses an internal combustion engine and a motor/generator as drive power sources.
2. Description of Related Art
FIG. 6 is a time chart showing a target motor rotational speed at the time of starting of an internal combustion engine, actual motor rotational speed, and a brake signal in a conventional hybrid vehicle.
Conventionally, at the time of starting the internal combustion engine, a motor is used to drive an internal combustion engine at its idling rotational speed. (Herein, the term xe2x80x9ca motorxe2x80x9d includes a motor/generator which regeneratively uses a motor as a generator). Subsequently, the internal combustion engine is started.
However, the conventional method described above requires a large driving torque because the motor must bring the internal combustion engine from a non-rotating state to a predetermined rotational speed. Once the internal combustion engine is started (ignited), it starts to rotate independently and no longer needs to be driven by the motor. Therefore, immediately after the internal combustion engine is started (ignited), as shown in FIG. 6, the output torque of the motor and the output torque of the internal combustion engine are combined. As a result, a peak PK which exceeds a target motor rotational speed TgtmtN is generated as an actual motor rotational speed mtN.
The peak PK which occurs in the rotational speed output from the combined drive sources causes a fluctuation in the rotational speed of the wheels of the vehicle, which fluctuation is felt as an undesirable impact by the driver. Therefore, it is desirable to suppress the peak PK to the extent possible.
Suppression of the peak PK becomes possible if, when the internal combustion engine is started (ignited), its start is accurately detected and the motor drive is stopped. For this reason, there is a demand for development of a control system capable of accurately detecting the point in time when the internal combustion engine has been started (ignited).
Accordingly, it is an object of the present invention to provide a control system and control method for a hybrid vehicle, which are capable of accurately detecting the start (ignition) of an internal combustion engine in the hybrid vehicle, and also suppressing the peak PK of actual motor rotational speed to the extent possible.
Accordingly, the present invention provides a control system for a hybrid vehicle which includes an internal combustion engine with a motor drivably connected thereto and, more particularly, a control system for starting the internal combustion engine in such a hybrid vehicle. The control system includes a motor control unit for driving the motor to cause the motor to rotate at a predetermined target motor rotational speed in preparation for starting the internal combustion engine. The control system further includes an internal combustion engine drive control unit for, responsive to the rotational speed of the internal combustion engine reaching the predetermined target rotational speed, outputting an ignition command to start the internal combustion engine. Still further, the control system includes an internal combustion engine ignition determining unit for, after output of the ignition command, determining the difference between the target rotational speed of the motor and the actual rotational speed of the motor and, if the determined difference exceeds a predetermined value, confirming that the internal combustion engine has been started.
In a preferred embodiment the control system further includes a target motor rotational speed setting unit which, responsive to confirmation of start of the internal combustion engine, resets the target rotational speed to a lower value. The resetting to a lower value may be based on the determined difference between the target rotational speed and the actual rotational speed of the motor.
Preferably, the control system is applied to a hybrid vehicle wherein the output shafts of the motor and internal combustion engine are directly connected.
In another aspect, the present invention provides a method for starting an internal combustion engine in a hybrid vehicle wherein the internal combustion engine is drivably connected to a motor. The method includes energizing the motor to cause the motor to drive the internal combustion engine to bring the rotational speed of the internal combustion engine from zero to a target rotational speed. Responsive to reaching the target rotational speed, fuel is injected into the internal combustion engine and an ignition command signal is issued. Subsequent output of the ignition command signal, the difference between the actual rotational speed of the motor and the target rotational speed is determined and the starting of the internal combustion engine is confirmed when the determined difference exceeds a predetermined value.
Preferably, the control method resets the target rotational speed to a lower value responsive to confirmation of start of the internal combustion engine and drives the motor at that lower, reset rotational speed. The target rotational speed may be reset based on the determined difference between the actual motor rotational speed and the target motor rotational speed.